Question

I have a pretty basic understanding of the GUI thread and the message loop, but I’m curious as to how that applies to one window starting a modal window. If I had to guess, I’d say that both windows are being run under the same GUI thread and that some parameter indicates that only events with the child window (the modal one) be executed, otherwise point out the modal window to the user.

This is simply a semi-educated guess and I accept that I may be wrong from square one. I’m not even sure if “GUI thread” is the right name for that thread, but people usually can guess what I’m talking about.

So in short, how do threads and modal windows get along together?

Answer 1

This maybe contradictory, but no, no new thread is started for the new windows. However, a new message loop is opened. This keeps messages flowing through Windows and avoids halting other applications.

Messages will arrive and may be dispatched to the owner window’s message loop. On the owner window, keyboard and mouse input have been disabled, but all other messages will be send through. Note from Hans Passant: actually, all top level windows of the same thread will be disabled this way.

As an example that you already touch on in your question, WM_PAINT is send through to the parent window. But also WM_TIMER, for instance. A message like WM_NCHITTEST will not be send through, as it is an input message. Nor will WM_KEYDOWN and similar.

This way, a messagebox can be moved, and the underlying owner gets neatly repainted, or a ticking clock still continues ticking.

Information partially from Rector and Newcomer’s Win32 Programming, page 752+, old, but still valuable and valid info. This info applies to DialogBox, DialogBoxParam, DialogBoxIndirect and DialogBoxIndirectParam as well as any of the ..Ex versions. Internally, these Win32 API functions are called by WinForms.